A curse is one of the seven known spell types and the worst kind of Dark Magic[1]. Curses are used with the intention of causing harm, control, or even death to the victim. Like hexes, Hogwarts students don't learn them in school. However, students studying Defence Against the Dark Arts learn how to deal with them using defensive magic.

Whilst Barty Crouch once said that the Killing Curse cannot be blocked, this is not strictly true. For example, one could obstruct its path with another object, such as Albus Dumbledore did when he animated the Fountain of Magical Brethren; further, it can be countered with a Stunning Spell and (in the case of Priori Incantatem) with the Disarming Charm. It can also be blocked by Sacrificial protection.